Resistance to infection by microorganisms makes use of non-specific functions (enzyme action, pH, epithelial wall) and of the adaptive immune responses of B and T lymphocytic cells.
The non-specific functions prevent invasion by the majority of attacking agents. However, when this first line of defense capitulates, the phagocytic-system comes into operation, destroys the infectious agents and stimulates the immunity functions conferred by the B and T cells.
Any abnormality, hereditary or acquired, of the phagocytic system has serious consequences, since even microorganisms which are normally of low pathogenicity evade it and trigger recurrent infections.
Moreover, any deficiency in the immune system itself, at T or B cell level, leads to an enhanced susceptibility to intra- or extracellular viral and bacterial infections. These deficiencies may be hereditary or acquired (e.g. AIDS=elective T cell deficiency). Most people suffering from these deficiencies are subject to infection by opportunistic organisms (bacteria, protozoa, and the like).
In all cases of immunosuppression, it is hence desirable that the phagocytic system is as effective as possible, in order to limit the consequences of external attack. Of secondary importance under normal conditions, phagocytosis takes on an essential character when the B and T immune response weakens.
Among cells associated with the immune response, the polymorphonuclear leukocytes are of special interest in the context of combating infections. These cells contain an enzyme, myeloperoxidase, whose microbicidal action is well documented. Polynuclear cells do not display any specificity with respect to an antigen, but play an essential part in the case of acute inflammation, with antibodies and the complement system, in the host's defense against microorganisms. Their main function is phagocytosis. During this process, the microorganisms are included in vacuoles (phagosomes) which fuse with the granules containing myeloperoxidase to form phagolysosomes. During phagocytosis, the enzymatic activity of the myeloperoxidase leads to the formation of HOCl, a potent bactericidal compound (hypochlorous acid); this activity requires H.sub.2 O.sub.2 (hydrogen peroxide), which appears in the polymorphonuclear cell when it is stimulated by various agents, and in particular by the immunological reactions induced by microorganisms. Hypochlorous acid is oxidizing in itself, but produces still more strongly oxidizing derivatives, chloramines. Finally, reacting with H.sub.2 O.sub.2 from which it is derived, hypochlorous acid produces an extremely oxidizing form of oxygen, singlet oxygen.